/Engine/Source/Runtime/Engine/Classes/EdGraph/EdGraphSchema.h
Takes the PinDescription and tacks on any other data important to the schema (things like the pin's type, etc.). The base one here just spits back the PinDescription.
virtual void ConstructBasicPinTooltip ( [UEdGraphPin](API\Runtime\Engine\EdGraph\UEdGraphPin) const & Pin, [FText](API\Runtime\Core\Internationalization\FText) const & PinDescription, [FString](API\Runtime\Core\Containers\FString) & TooltipOut ) const